Psychophisical studies frequently identify sex-based differences in sensory capability. In most cases the mechanistic basis of these differences is poorly understood. This is particularly evident when one considers human olfactory capability. Women are reportedly more sensitive to odors and and to have better odor identification abilities than men but the physiological basis is unknown. The lack of an appropriate animal model has hampered physiological and anatomical investigations of the underlying mechanisms. While characterizing the olfactory capabilities of zebrafish we discovered that the response of females to all general feeding and social odorants tested was larger thatn the corresponding male response. Understanding the mechanistic basis of these sex-based differences in general olfactor sensitivity (as opposed to pheromone sensitivity) is the focus of current application. Three basic questions direct our investigation. 1) Do the significant sex-based differencesin sensitivity observed in response to a few odorants generalize to most odorants including putative pheromones? 2) What influence does reproductive status play in determining olfactory sensitivity? 3) Can physiological and/or anatomical differences in the olfactory organs of the sexes account for the differences in sensitivity? Contemporary electrophysiological and Ca 2+ imaging techniques will be used to determine if basic biophysical properties of individual olfactory receptor neurons are influenced by sex. An activity dependent immunostaining procedure and standard morphometric techniques will be used to estimate the density of competent odor-sensitive ORNs. The zebrafish is oneof a very few vertegrate models where a complete molecular picture of embryonic development including development of the olfactory system is likely to emerge. Consequently, experimenets intended to address fundamental questions about sex-based differences in olfactory processing will also provide an essential physiological context to aid in interpretationof developmental and molecular genetic data that are being obtained in this lab and others.